Iran is fanning the flames of sectarian violence in Iraq, providing training and weapons to militias and extremist groups opposed to the country's unity, the US ambassador to Iraq claimed yesterday.

Speaking at a news conference in Baghdad, Zalmay Khalilzad said Washington was not seeking to exploit the standoff with Iran over its controversial nuclear programme, "but we don't want Iranian interference in Iraq, providing arms and training the militias and forces that are hostile to this new Iraq".

He said Baghdad and Tehran enjoyed a good working relationship, but "Iran has another policy as well, which is to work with the militias and extremist groups, and provide training and weapons, direct or indirect".

The ambassador, a fluent Farsi speaker known for his hands-on approach, added that Iran's recent demand that Britain pull its troops out of the southern Iraqi city of Basra was "uncalled-for interference".

"Basra is Iraqi territory the last time I checked the map," he said.

Mr Khalilzad did not name the groups he said were receiving Iranian help. But Iraqi Sunnis are suspicious of the links between Iran and senior Shia political leaders in Iraq. Shia militias such as the Badr brigade, loyal to the Supreme Council for Islamic Revolution in Iraq, and Moqtada al-Sadr's al-Mahdi army, have flourished in the vacuum that prevailed over much of Iraq following the fall of Saddam.

The Badr forces are said to run anti-Sunni death squads from inside the country's interior ministry. The Mahdi militia, meanwhile, is accused of kidnapping and murdering political rivals and the arbitrary execution of Sunni Muslims in what they claim is a legitimate hunt for Ba'athists and Wahhabists. Sunni extremist groups and former Ba'ath party loyalists are blamed by many Iraqi Shia for the relentless wave of suicide bombs targeting their community.

Tehran denies it is trying to destabilise Iraq, arguing it is the continued US presence that is fuelling the strife.

The violence continued yesterday, with three bombs that killed at least 19 people. The deadliest attack occurred in the predominantly Shia district of Kadhimiya in Baghdad, where a suicide bomber blew himself up aboard a bus, killing at least 12 people. Four civilians and a policeman were killed in Mosul by a bomb planted inside a restaurant. Two people died in a car bomb in south-east Baghdad.

The renewed violence came as Iraqi politicians struggled to make headway in forming a government of national unity. But more than two months after the landmark December elections, the main Shia, Kurdish and Sunni blocs remain at odds over issues such as federalism and control of the new security structures.

At his news conference Mr Khalilzad warned Iraqi leaders Washington would not tolerate sectarianism or militias in the new government and its security forces. "The United States is investing billions of dollars into these forces, military and police forces of Iraq. American taxpayers expect their money to be spent properly."

A western diplomat, who requested anonymity, later said: "The US is trying to allow a political order to emerge in Iraq rather than impose one. But it will not sit by and let Iran dominate, nor will it allow the country to fall back into dictatorship."